To setup an auto-responder you will need to be logged into the control panel. Now click on the envelope icon, beside the user you wish to set an auto-responder for. The page that loads up will have three boxes. Select the bottom box, which is for "vacation response" Type the message in the box that you want it to send. Note that by enabling this feature for a user that receives mail for an alias as well, the auto-responder will be sent out for those.